Control system for turbine driven positive displacement pumps



1961 D. R. TROWBRIDGE ET AL 2,997,846

CONTROL SYSTEM FOR TURBINE DRIVEN POSITIVE DISPLACEMENT PUMPS Filed July9, 1957 2 Sheets-Sheet l 1961 D. R. TROWBRIDGE ET AL 2,997,846

CONTROL SYSTEM FOR TURBINE DRIVEN POSITIVE DISPLACEMENT PUMPS Filed July9, 195'? 2 Sheets-Sheet 2 NIL mmnn MW United States Patent M 2,997,846CONTROL SYSTEM FOR TURBINE DRIVEN POSITIVE DISPLACEMENT PUMPS David R.Trowbridge, Hutton Mount, near Brentwood, Norman Moss, London, andMichael J. Broad, Enfield, England, assignors to The Plessey CompanyLimited, Ilford, England, a British company Filed July 9, 1957, Ser. No.670,832 2 Claims. (Cl. fill-39.28)

This invention relates to means for automatically controlling the speedof the turbine in a turbine-driven positive-displacement pump undervarying load conditions of a hydraulic system fed by the pump. InBritish Patent specification No. 847,372 there is described aturbinedriven positive-displacement pump for operation under varyingload conditions of a hydraulic system fed by the pump, wherein anauxiliary pump is driven by the turbine, and the pressure produced bythis auxiliary pump acts upon a pressure-responsive member in oppositionto the pressure produced by the positive displacement pump; an automaticcontrol means acts to increase the power input to the turbine when thepressure produced by the positive-displacement pump increases and/or thespeed of the turbine decreases, and to decrease said power in put in theconverse case.

In one form of the apparatus described and claimed in the said Britishspecification the turbine driving the positive displacement pump is ahot-gas turbine, and the auxiliary pump serves to supply fuel to acombustion chamber feeding the turbine, the pressure-responsive devicebeing arranged to control a spill valve to deflect and return a variableportion of the flow from the fuel pump. The present invention has for anobject so to modify the last-mentioned construction as to make itsuitable for the use of a centrifugal pump as the fuel pump.

Another object is to provide an improved automatic control valve of highsensitivity for such automatic control system, which will eifect a rapidreduction in fuel supply when the delivery pressure of the positivedisplacement pump drops below a predetermined value, a rapid decrease infuel supply when the delivery pressure of the positive-displacement pumpdrops below a predetermined value, a rapid decrease in fuel supply whenthe delivery pressure reaches a second higher value, and in either casea progressive decrease or increase in the fuel supply thus establishedwhen and according as the turbine speed rises above, or falls below, apredetermined value.

According to the present invention a metering orifice is interposed inthe fuel-pump delivery line between the fuel pump and the spill valve,the valve device being preferably so constructed as to apply fuel-pumppressure direct to the appropriate control surface through a separatepassage.

FIG. 1 of the accompanying drawing illustrates one simple embodiment ofthe invention, while FIGS. 2 and 3 of the drawings accompanying thespecification are two modifications of the valve assembly shown in FIG.1.

Referring now to the drawings, the complete unit illustrated in FIG. 1comprises a combustion chamber 2, to which liquid fuel is fed, eitherbefore or after vapourisation, by a centrifugal fuel pump 30, and whichsupplies a How of combustion gases to a gas turbine 1, which drives boththe centrifugal fuel pump 30 and a gear pump 4 which constitutes theturbine-driven positive-displacement pump and is interposed between anintake conduit 6 from a supply tank 32 and a delivery conduit which isconnected through a nonreturn value 33 to a hydraulic accumulator 34which is also connected by a pressure line 38 to a hydraulic load system(not shown). The accumula- Patented Aug. 29, 1961 ICC tor 34 is equippedwith a pressure-responsive actuator 35 which, when the accumulatorpressure exceeds a predetermined value, operates an unloading valve 36to open a by-pass line 37 and thus establish a direct connection fromthe delivery outlet 5 of the gear pump 4 to the tank 32, therebyunloading the pump 4. Return flow from the accumulator 34 to tank 32 isprevented in these circumstances by the non-return valve 35. Thiscombination of a hydraulic accumulator with a nonreturn valve and withautomatic unloading means is well known in the art and is accordinglynot claimed as novel. A metering orifice 31 is interposed between theparts 12 and 13 of a line leading from the centrifugal pump 30 to thecombustion chamber 2, and this metering orifice 31 is followed in seriestherewith by a chamber 10a which is equipped with a spill valve 18, 18aand a spill passage 17 by which excess fuel is returned to the fuelreservoir (not shown). The valve element 18a is a slim cone, connectedby means of a stern 18b to a bellows 9 which is situated within thechamber 10a; the interior of the bellows communicates with the deliveryside of the gear pump 4 by a passage 7. 11 is a spring opposing thepressure in the line 7. This spring has been shown as a separate elementbut may in practice alternatively be constituted by the resilience ofthe bellows 9.

When the device is in operation and turbine speed rises, the pressure ofcentrifugal pump 30 will rise, tending to deliver more fuel throughmetering aperture 31 and passage 13 to the combustion chamber 2. As soonhowever as the pressure in chamber 10a begins to increase, thisincreased pressure will act upon the bellows 9 to open, or increase theopening of, valve 18, thereby allowing some of the fuel deliveredthrough line 12 to be returned to the reservoir through spill passage17. Since this spill flow, as well as the flow to the combustion chamber2, is supplied by the pump 30 and thus constitutes an additional flowthrough the metering orifice, it causes the pressure drop across saidorifice to increase. An increased pressure drop resulting from theincreased flow through metering orifice 31, lowers the pressure in thechamber 10a, relative to the pressure produced by the centrifugal pumpwhich at a given speed tends to be constant over a wide range ofdelivery, and thus counteracts the tendency of the flow to chamber 2 toincrease. When the delivery pressure of gear pump 4 drops, as it willwhen a fiuid pressure reservoir in the consumer system is fully chargedand pump 4- is therefore unloaded, this will cause the valve 18 to openwide, thus allowing maximum spillage, reducing the fuel supplied throughline 13 to the minimum required for maintaining combustion and operationof the turbine under idling conditions.

FIG. 2 is a sectional elevation of a valve assembly correspondingsubstantially to that of FIG. 1. The same reference numbers as in FIG. 1are employed for the connections, except that the connections to lines 7and 13 are respectively referenced at 7a and 13a, and that the fuel-pumpconnection 12 has been replaced by two parallel connections 12a and 12b.The connection 12a leads through a metering orifice 22 to a spill valveand via the pipe connection 13, to the combustion chamber, while theconnection 12b is unrestricted and serves to apply the delivery pressureof the fuel pump to the pressureresponsive member controlling the spillvalve. It will be appreciated that, due to the provision of the meteringorifice 22, the variation of the pressure at the fuel pump in responseto a variation in turbine speed is much greater than the variation ofpressure of the spill valve, so that the provision of the unrestrictedsecond connection 12b renders the control more sensitive.

The bellows 9 of FIG. 1 has been replaced by two oppositely actingsliding piston members of different areas, namely a piston 19 ofrelatively large area, on which the delivery pressure of thecentrifugal-type fuel pump acts through pipe 12b and a second piston 20of much smaller area to which the delivery pressure of thepositive-displacement pump 4 of the hydraulic system is applied throughpipe 7.

Fuel for feeding the combustion chamber 2 is derived (through pipe 13)from a control chamber b, to which it is supplied through pipe 12a, andmetering orifice 22, while a spill port 21 leading to spill passage 17is controlled by a shuttle-type slide valve 23. The latter is integralwith the piston 19 at one end, while the piston 24 assisted by a spring24, acts on its other end. The position illustrated is the off-loadposition, in which the hydraulic accumulator 34 supplied by pump 4 isfully charged and pump 4 is unloaded in a well-known manner.

The hydraulic pressure in line 7 has therefore fallen to a smallfraction of the working pressure, whereupon the pressure produced by thefuel pump 30 and admitted to piston 19 through the unrestricted passage12b, has rapidly moved the shuttle valve 23 to a position, determined bythe spring 24, in which the spill port 21 is throttled only sufficientlyto ensure the delivery of the fuel necessary for maintaining the normalturbine speed under idling conditions.

FIG. 3 shows a modification of the valve just described, in which thespring 24a that biases the shuttle valve against the action of the fuelpressure is interposed between the shuttle valve 23a and the piston 20aon which the delivery pressure of the positive-displacement pump 4 acts.The piston 20a is for this purpose provided with a thrust collar 25. Thereferences used are the same as in FIGURE 2, except for the addition ormodification of an index letter a or b where an element has beenmodified, and the addition of some references for additional elements.The operation of the valve assembly is similar to that of FIG. 2, exceptthat the device is insensitive to any small change in the deliverypressure of the hydraulic pump 4. This is achieved due to the fact thatthe hydraulic piston 20a will take up either one or the other of twoextreme positions determined respectively by stops 23c and 23d,according as pump 4 works under load or is unloaded. Accordingly, thevalve 23 passes a large or small volume of fuel, mainly according to theloading of the hydraulic system, but any small changes in load requiringfuel variation in order to maintain the system in equilibrium are sensedthrough the fuel delivery pressure as the turbine speed rises and falls.The fuel delivery pressure acts on the valve similarly as describedabove, trimming the fuel requirements about one or the other of two meanvalues alternatively provided by the valve according as the hydraulicpump works under load or under idling conditions.

In each of the embodiments of FIGS. 2 and 3 stops 23b (FIG. 2) or 23c,23d, (PEG. 3) may be provided if desired for limiting the movement ofthe shuttle valve 23 or 23a in one or both directions.

What is claimed is:

1. In or for an automatic fuel control system for a hotgas turbinedriving a positive displacement pump working against variable pressurewherein the system includes a dynamic fuel pump delivering fuel to thehot-gas turbine through a delivery line that includes a metering orificeand an automatic control valve operable to divert a variable part of thefuel in said line to a spill line for varying the quantity of the fueldelivered to the turbine, the construction of the automatic controlvalve, comprising in combination a valve housing having two co-axialbores separated by an interstice communicating with atmosphere, a pistonin each said bore, a thrust spring interposed between said pistons,abutment means in one of the pistons which cooperate with the otherpiston to limit the mutual approach of the piston, means forestablishing communication of the outer ends of said bores respectivelywith the variable delivery pressure of the positive-displacement pumpand with the fuel, delivery line between the fuel pump and the meteringorifice, means positively limiting the inward and outward strokes of oneof the pistons, and a valve element connected at least operatively tothe other piston and operative to increase the fiow diverted to thespill line upon inward movement and decrease the flow diverted to saidspill line upon outward movement of said other piston.

2. An automatic control valve as claimed in claim 1, wherein said valveelement is a valve piston fixed coaxially to said other piston andsealingly movable in the same bore, the valve housing further having apassage provided with means for establishing communication between saidpassage and the fuel line from the fuel pump, said passage leading fromthe fuel line to said bore between the innermost position of said otherpiston and the outermost position of said valve piston, the meteringorifice being arranged in said passage, and a spill passage from saidbore and co-operating with said valve piston in said bore for control,by said valve piston, of the flow from said passage through said bore tothe spill passage, and a branch passage communicating with said passagebetween said communication and establishing means and the meteringorifice and having means for connection through the fuel line to the hotgas turbine.

References Cited in the file of this patent UNITED STATES PATENTS2,581,275 Mock Jan. 1, 1952 2,670,599 Davies et a1. Mar. 2, 19542,808,702 Dotson Oct. 8, 1957

